Method for forming an acid-treated fly ash activated carbon

ABSTRACT

A process for making a modified, mesoporous activated carbon material from waste oil fly ash. The process involves a physicochemical treatment of a raw waste oil fly ash sample, where the sample is initially refluxed in an acid solution, then activated at about 1000° C. and in the presence of carbon dioxide. The activated carbon may be further functionalized with carboxylic and/or amine groups by refluxing the activated carbon in a second acid solution and/or an ammonia solution. The activated carbon, as prepared, has a BET surface area of 30-400 m 2 /g, a total pore volume of 0.25-0.50 cm 3 /g and an average pore size of 40-100 Å. A method for removing hydrogen sulfide from natural gas with the modified activated carbon is also provided.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of Ser. No. 14/920,290, now allowed, having a filing date of Oct. 22, 2015.

BACKGROUND OF THE INVENTION

Technical Field

The present invention relates to a process for preparing activated carbon. More specifically, the present invention relates to a process for preparing modified, mesoporous activated carbon by physicochemical treatment of waste oil fly ash. The prepared activated carbon material is suitable for hydrogen sulfide gas removal applications.

Description of the Related Art

The “background” description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, are neither expressly or impliedly admitted as prior art against the present invention.

Hydrogen sulfide (H₂S) is a major pollutant where the presence thereof in natural gas results in major corrosion and environmental problems. Hydrogen sulfide is toxic and is a most harmful toxin gas for human and animals. It becomes fatal when its concentration exceeds 500 ppm [Y. Elsayed, M. Seredych, A. Dallas, T. J. Bandosz, Desulfurization of air at high and low H₂S concentrations, Chem. Eng. J. 155 (2009) 594-602; P. Forzatti, L. Lietti, Catalyst deactivation, 52 (1999) 165-181; W. J. Powers-Schilling, Olfaction: Chemical and psychological consideration, in: Nuisance Concern Animal. Management: Odor and Flies, Gainesville, Fla., 1995; Y. Xiao, S. Wang, D. Wu, Q. Yuan, Catalytic oxidation of hydrogen sulfide over unmodified and impregnated activated carbon, Sep. Purif. Technol. 59 (2008) 326-332—each incorporated herein by reference in its entirety]. On the other hand, the presence of H₂S at concentrations higher than 5.5 mg/m³ in natural gas leads to sulfur stress cracking that reduces life of processing and handling equipment. To overcome these adverse effects, several commercial technologies have been used for H₂S removal from natural gas. The amine sweetening process is widely used in industries to lower the concentration of H₂Sto the target level imposed by customers and downstream processors [R. Álvarez-Cruz, B. E. Sánchez-Flores, J. Torres-González, R. Antaño-López, F. Castañeda, Insights in the development of a new method to treat H₂S and CO₂ from sour gas by alkali, Fuel. 100 (2012) 173-176; M. Tagliabue, C. Rizzo, N. B. Onorati, E. F. Gambarotta, A. Carati, F. Bazzano, Regenerability of zeolites as adsorbents for natural gas sweetening: A case-study, Fuel. 93 (2012) 238-244—each incorporated herein by reference in its entirety]. This method is costly in term of heat required for regeneration and also produces unwanted by-products. Other treatment processes, such as membrane separation and biological treatment, either suffer from low selectivity or they are not feasible at the larger scales [J. I. Huertas, N. Giraldo, S. Izquierdo, Removal of H₂S and CO₂ from Biogas by Amine Absorption, in: D. J. Markoa (Ed.), Mass Transfer in Chemical Engineering Process, In Tech Europ, 2011, pp. 132-150—incorporated herein by reference in its entirety].

Adsorption, on the other hand, can be used to capture H₂S at trace levels with relatively low cost of adsorbent. This process becomes especially attractive option when an adsorbent material, such as waste oil fly ash, is available in large quantities and for low cost. Waste oil fly ash is byproduct of many industrial and power generation plant operations [M. Sharma, C. Guria, A. Sarkar, A. K. Pathak, Recycle of waste fly ash: A rheological Investigation, Int. J. Sci. Environ. Technol. 1 (2012) 285-301—incorporated herein by reference in its entirety]. Waste oil fly ash usually causes environmental pollution problems and requires safe disposal. Therefore, utilization of waste oil fly ash in removing H₂S is expected to solve more than one environmental problem.

Since waste oil fly ash is pozzolanic in nature, it contains mainly unburned carbon (˜80%) with some inorganic oxides like SiO₂, Fe₂O₃, Al₂O₃, and CaO and traces of heavy metals [R. Shawabkeh, M. J. Khan, A. a. Al-Juhani, H. I. Al-Abdul Wahhab, I. a. Hussein, Enhancement of surface properties of oil fly ash by chemical treatment, Appl. Surf. Sci. 258 (2011) 1643-1650—incorporated herein by reference in its entirety]. According to a survey conducted by American Coal Ash Association (ACAA) over 100 million tons of coal combustion products were produced in 2012, where only 38% of total coal combustion products were used beneficially [American Coal Ash Association, Coal Combustion Product (CCP) Production & Use Survey Report, 2012—incorporated herein by reference in its entirety]. However, utilization rate of fly ash has increased greatly in China reaching up to 67% in 2010 compared to 20% rate in 1999 [Z. Tang, S. Ma, J. Ding, Y. Wang, S. Zheng, Current status and prospect of fly ash Utilization in China, in: 2013 World Coal Ash Conference, 2013, pp. 22-27—incorporated herein by reference in its entirety]. The majority of fly ash is used in blended cements, filler for metal matrix composites, as raw material for metal recovery and as filler for polymers [RA. Shawabkeh, Adsorption of chromium ions from aqueous solution by using activated carbo-aluminosilicate material from oil shale., J. Colloid Interface Sci. 299 (2006) 530-6; T. P. D. Rajan, R. M. Pillai, B. C. Pai, K. G. Satyanarayana, P. K. Rohatgi, Fabrication and characterisation of Al-7Si—0.35Mg/fly ash metal matrix composites processed by different stir casting routes, Compos. Sci. Technol. 67 (2007) 3369-3377; R. Navarro, J. Guzman, I. Saucedo, J. Revilla, E. Guibal, Vanadium recovery from oil fly ash by leaching, precipitation and solvent extraction processes., Waste Manag. 27 (2007) 425-38; Q. Zeng, K. Li, T. Fen-Chong, P. Dangla, Surface fractal analysis of pore structure of high-volume fly-ash cement pastes, Appl. Surf. Sci. 257 (2010) 762-768; M. a. Al-Ghouti, Y. S. Al-Degs, A. Ghrair, H. Khoury, M. Ziedan, Extraction and separation of vanadium and nickel from fly ash produced in heavy fuel power plants, Chem. Eng. J. 173 (2011) 191-197; K. T. Hideaki Tokuyama, Susumu Nii, Fumio Kawaizumi, Characterization of Al—Cu alloy reinforced fly ash metal matrix composites by squeeze casting method, Int. J. Engg. Sci. Technol. 5 (2013) 71-79; A. K. Senapati, A. Bhatta, S. Mohanty, P. C. Mishra, B. C. Routra, An extensive literature review on the usage of fly ash as a reinforcing agent for different matrices, Int. J. Innov. Sci. Mod. Eng., vol. 2, no. 3(2014) 4-9—each incorporated herein by reference in its entirety]. Recently, oil fly ash has gained particular attention as potential adsorbent for several adsorbate due to its high carbonaceous content and low cost [A. L. Yaumi, R. Aww. K. ShaWabkeh, ilbnesllvaleed A. Hussem, U.S. Pat. No. 8,545,781 BI, 2013; A. L. Yaumi, I. a. Hussien, R. a. Shawabkeh, Surface modification of oil fly ash and its application in selective capturing of carbon dioxide, Appl. Surf. Sci. 266 (2013) 118-125; B. Rubio, M. T. Izquierdo, Coal fly ash based carbons for SO₂ removal from flue gases., Waste Manag. 30 (2010) 1341-7; CN103626174A; US20140197020A1; KR996431B1—each incorporated herein by reference in its entirety]. Izquierdo et al studied SO₂ removal using activated carbon (AC) produced from oil agglomerated coal fly ash. They compare the adsorption efficiency of anthracite coal based fly ash activated carbon with bituminous-lignite blended coal fly ash activated carbon and concluded that the latter is superior with 28 mg/g uptake capacity [B. Rubio, M. T. Izquierdo, Coal fly ash based carbons for SO2 removal from flue gases., Waste Manag. 30 (2010) 1341-7—incorporated herein by reference in its entirety]. With some chemical treatment, the porosimetric characteristics of ash may be enhanced to obtain a high surface area AC. Thus obtained, AC can be used to remove pollutants from flue gas. Yaumi et al. used treated oil fly ash for the adsorption of CO₂ under different flow conditions. A removal capacity of 240 mg CO₂/g-treated OFA was achieved. The interactions between CO₂ and ash surface were reported to be endothermic in nature [A. L. Yaumi, R. Aww. K. ShaWabkeh, ilbnesllvaleed A. Hussem, U.S. Pat. No. 8,545,781 B1, 2013; A. L. Yaumi, I. a. Hussien, R. a. Shawabkeh, Surface modification of oil fly ash and its application in selective capturing of carbon dioxide, Appl. Surf. Sci. 266 (2013) 118-125—each incorporated herein by reference in its entirety].

Various treatment strategies could be implemented to increase the porosity and create some ordering of structure like in the synthesis of zeolites [A. Alastuey, E. Herna, X. Querol, N. Moreno, J. C. Uman, F. Plana, Synthesis of zeolites from coal fly ash: an overview, Int. J. Coal Geol. 50 (2002) 413-423; M. Wdowin, M. Franus, R. Panek, L. Badura, W. Franus, The conversion technology of fly ash into zeolites, Clean Technol. Environ. Policy. (2014). M. Visa, A. Duta, TiO₂/fly ash novel substrate for simultaneous removal of heavy metals and surfactants, Chem. Eng. J. 223 (2013) 860-868; M. M. Maroto-valer, Z. Lu, Y. Zhang, Z. Tang, Sorbents for CO₂ capture from high carbon fly ashes, Waste Manag. 28 (2008) 2320-2328—each incorporated herein by reference in its entirety]. For example, external heating of fly ash with acid mixture involves various sulfonation and nitrification reactions including the formation of phosphate functional groups. As a result oxides of sulfur and nitrogen and carbon dioxide are released during the chemical activation process [B. Bournonville, A. Nzihou, P. Sharrock, G. Depelsenaire, Stabilisation of heavy metal containing dusts by reaction with phosphoric acid: study of the reactivity of fly ash., J. Hazard. Mater. 116 (2004) 65-74; R. A. Shawabkeh, Synthesis and characterization of activated carbo-aluminosilicate material from oil shale, Microporous Mesoporous Mater. 75 (2004) 107-114—each incorporated herein by reference in its entirety]. Treatment of fly ash with acids also introduces hydrophilic groups like carboxylic and hydroxyl groups on the surface of ash [E. D. Dimotakis, M. P. Cal, J. Economy, M. J. Rood, S. M. Larson, Chemically treated activated carbon cloths for removal of volatile organic carbons from gas streams: evidence for enhanced physical adsorption., Environ. Sci. Technol. 29 (1995) 1876-80—incorporated herein by reference in its entirety].

Several kinds of carbon based adsorbents have been employed to capture H₂S from gas stream. These adsorbents include agro-based activated carbon, coal based and impregnated activated carbon [J. Kazmierczak, P. Nowicki, R. Pietrzak, Sorption properties of activated carbons obtained from corn cobs by chemical and physical activation, Adsorption. 19(2013) 273-281; H. S. Choo, L. C. Lau, A. R. Mohamed, K. T. Lee, Hydrogen sulfide adsorption by alkaline impregnated coconut shell activated carbon, J. Eng. Sci. Technol. 8 (2013) 741-753; D. Choi, J. Lee, S. Jang, B. Ahn, D. Choi, Adsorption dynamics of hydrogen sulfide in impregnated activated carbon bed, Adsorption. 14 (2008) 533-538; A. Bagreev, J. Angel Menendez, I. Dukhno, Y. Tarasenko, T. J. Bandosz, Bituminous coal-based activated carbons modified with nitrogen as adsorbents of hydrogen sulfide, Carbon. 42 (2004) 469-476—incorporated herein by reference in its entirety].

In view of the foregoing, there remain numerous, ongoing efforts directed towards development of processes for preparing activated carbon with new raw materials. The present disclosure provides a process for manufacturing modified, mesoporous activated carbon where the carbon activation procedure is relatively simple and straightforward, and the surface area of the activated carbon can be dramatically increased during the manufacturing process.

BRIEF SUMMARY OF THE INVENTION

In a first aspect, there is provided a process for preparing an activated carbon material. The process comprises refluxing a waste oil fly ash powder with a first acid solution to form a solid residue and heating the solid residue at 900-1050° C. and purging the solid residue in a flow of carbon dioxide to form the activated carbon material.

In certain embodiments, the first acid solution comprises one or more acids selected from the group consisting of sulfuric acid, nitric acid and phosphoric acid. In one embodiment, the first acid solution comprises two acids at a volume ratio of 1-13:1-13. In another embodiment, the first acid solution comprises three acids at a volume ratio of 1-8.5:1-8.5:1-8.5.

In one or more embodiments, the refluxing is carried at 100-150° C. for 3-6 h and at a concentration of 10-100 g of the waste oil fly ash powder per liter of the first acid solution.

In one or more embodiments, the heating is carried out for 30-90 min.

In one or more embodiments, the purging is carried out for 15-45 min, at 1-5 bar and at a carbon dioxide flow rate of 0.5-2.0 L/min.

In some embodiments, the solid residue is not chemically modified during the purging.

In one embodiment, the solid residue has a pH value of 5.5-7.5.

In certain embodiments, the process for preparing the activated carbon material further comprises contacting and refluxing the activated carbon material with a second acid solution or an ammonia solution to attach carboxylic groups or amine groups onto the surface of the activated carbon material. In one embodiment, the second acid solution is a nitric acid solution. In one or more embodiments, the contacting and the refluxing are carried out at 80-120° C. for 3-6 h and at a concentration of 50-200 g of the activated carbon material per liter of the second acid solution or the ammonia solution.

In some embodiments, the refluxing, the heating and the purging increase the BET surface area of the activated carbon material, increase the total pore volume of the activated carbon material and reduce the average pore size of the activated carbon material.

In a second aspect, the present disclosure provides an activated carbon material prepared by the process according to the first aspect of the disclosure. The activated carbon material has a BET surface area of 30-400 m²/g, a total pore volume of 0.25-0.50 cm³/g and an average pore size of 40-100 Å.

In a third aspect, the present disclosure relates to a method for removing hydrogen sulfide from a gaseous stream. The method comprises contacting the gaseous stream with the activated carbon material according to the second aspect of the disclosure.

In one or more embodiments, the gaseous stream is a natural gas stream.

In certain embodiments, the activated carbon material has a concentration of 0.01-0.05 g per ppm of the hydrogen sulfide present in the gaseous stream.

In one embodiment, the activated carbon material is affixed to a packed bed of an adsorption column and the gaseous stream is passed through the packed bed at 0.1-1.0 L/min at atmospheric pressure and at ambient temperature.

In one embodiment, the activated carbon material adsorbs the hydrogen sulfide and has a hydrogen sulfide adsorption capacity of 0.10-0.35 mg of hydrogen sulfide per gram of the activated carbon material.

In one embodiment, the activated carbon material has a regeneration efficiency of 40-90%.

The foregoing paragraphs have been provided by way of general introduction, and are not intended to limit the scope of the following claims. The described embodiments, together with further advantages, will be best understood by reference to the following detailed description taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the disclosure and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:

FIG. 1 is a schematic diagram of an experimental setup for H₂S breakthrough adsorption/desorption measurements (PI: Pressure Indicator; FI: Flow Indicator; HI: Humidity Indicator).

FIG. 2A shows the pore size distribution of waste oil fly ash samples before treatment.

FIG. 2B shows the pore size distribution of waste oil fly ash samples after treatment.

FIG. 3A is an SEM image of the surface of a waste oil fly ash raw sample.

FIG. 3B is an SEM image of the surface of an acid-treated waste oil ash sample.

FIG. 3C is an SEM image of the surface of a CO₂-activated waste oil ash sample.

FIG. 3D is an SEM image of the surface of a NH₄OH-functionalized treated waste oil ash sample.

FIG. 3E is an SEM image of the surface of a HNO₃-functionalized treated waste oil ash sample.

FIG. 4 shows the adsorption-desorption breakthrough curves for a raw waste oil fly ash sample and the sample after acid treatment (Pressure=1 atm, Temperature=22° C., Relative Humidity=20%, Flow rate=0.4 L/min).

FIG. 5 shows the adsorption-desorption breakthrough curves for a raw waste oil fly ash sample after physic-chemical treatment (Pressure=1 atm, Temperature=22° C., Relative Humidity=20%, Flow rate=0.4 L/min).

FIG. 6 shows the adsorption-desorption breakthrough curves of physic-chemically treated waste oil fly ash after functionalization with HNO₃ and NH₄OH (Pressure=1 atm, Temperature=22° C., Relative Humidity=20%, Flow rate=0.4 L/min).

DETAILED DESCRIPTION OF THE EMBODIMENTS

Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views.

In the present disclosure, there are provided processes for manufacturing modified, mesoporous activated carbon by physicochemical treatment of waste oil fly ash.

As used herein, the term “waste oil fly ash” or “waste oil flue ash” or “oil fly ash” or “oil flue ash” or OFA refers to a waste residue generated in combustion of heavy fuel oil, for example, in power generation plants. Waste oil fly ash comprises fine ash particles that rise together with flue gases. Waste oil fly ash has a carbon content of at least 70% by weight, preferably 70-75%, 75-80%, more preferably 80-85% 85-90%, 90-95%, with the rest of the components being primarily metals or semimetals such as but not limited aluminum, magnesium, calcium, vanadium, nickel, copper, zinc chromium, cobalt, lead, manganese, mercury, molybdenum, selenium strontium, thallium, arsenic, beryllium, boron, cadmium, as well as silicon and/or silica, sulfur, oxygen, organic compounds including but not limited to dioxins or polychlorinated dibenzodioxins (PCDDs) and polyaromatic hydrocarbons (PAHs). In certain embodiments, the oxygen content and the sulfur content of the waste oil fly ash each range 2-25% by weight, preferably 3-20%, more preferably 4-15%, even more preferably 5-10%. In some embodiments, each of the elements aluminum, magnesium, calcium, vanadium, nickel, copper, zinc and silicon constitutes 0.01-5.0% of the waste oil fly ash by weight, preferably 0.01-2.5%, preferably 0.02-2.0%, preferably 0.05-1.0%, more preferably 0.1-0.5%.

As used herein, a “mesoporous activated carbon” refers a porous activated carbon material where the average pore size is within the range of 40-100 Å of 4-10 nm. Accordingly, the term “mesopore” as used herein refers to a pore having a diameter of 40-100 Å. The term “micropore” refers to a pore having a diameter of less than 40 Å. The term “macropore” refers to a pore having a diameter that exceeds 100 Å.

After collection, a raw waste oil fly ash sample can be advantageously subjected to pretreatment such as drying and dry-sieving with a 35-400 μm mesh, preferably 35-250 μm, more preferably 35-150 μm, even more preferably 35-105 μm, most preferably 35-75 μm.

The waste oil fly ash powder is then mixed with an acid solution to a final concentration of 10-100 g/L (g of waste oil fly ash powder per volume of acid solution), preferably 15-75 g/L, more preferably 25-50 g/L and mixed well. The waste oil fly ash-acid suspension is then refluxed at 100-150° C., preferably 115-135° C. for 3-6 h or until sufficiently treated. In one embodiment, the reflux condition is 125° C. and 4 h. The acid solution contains one or more inorganic or mineral acids selected from nitric acid, sulfuric acid, phosphoric acid, perchloric acid, hydrobromic acid, hydrochloric acid, hydroiodic acid, hydrofluoric acid and boric acid. In certain embodiments, the acid solution contains two or three mineral acids selected from nitric acid, sulfuric acid and phosphoric acid, at volume ratios of 1-13:1-13 (35-65 vol. %:35-65 vol. %) for two acids or 1-8.5:1-8.5:1-8.5 (15-42.5 vol. %:15-42.5 vol. %:15-42.5 vol. %) for three acids. However, due to exceedingly strong oxidizing capacities which can be detrimental towards the mass and other structural properties of the final activated carbon product, a mixture consisting of nitric acid and sulfuric acid is avoided. In one embodiment, the acid solution contains 20 vol. % sulfuric acid, 40 vol. % nitric acid and 40 vol. % phosphoric acid. In other embodiments, the acid solution contains 100 vol. % phosphoric acid; 40 vol. % nitric acid and 60 vol. % phosphoric acid; 100 vol. % nitric acid; 40 vol. % sulfuric acid and 60 vol. % phosphoric acid; 40 vol. % sulfuric acid, 20 vol. % nitric acid and 40 vol. % phosphoric acid; or 100 vol. % sulfuric acid. In alternative embodiments, one or more organic acids may be used to prepare the acid solution. Examples of organic acids include but are not limited to formic acid, acetic acid, propionic acid, butyric acid, valeic acid, caproic acid, oxalic acid, lactic acid, malic acid, citric acid, carbonic acid, benzoic acid, phenol, uric acid, carboxylic acids and sulfonic acid. In another embodiment, the acid solution may contain mixtures of mineral and organic acids at volume ratios defined above.

A solid residue is obtained at the end of the acid treatment, which may be filtered and rinsed repeatedly with distilled water, then dried. The purpose of rinsing the solid residue with high amounts of distilled water is to wash away the acid solution and to prepare the waste oil fly ash sample for physical inactivation by increasing the pH of the solid residue to 5.5-7.5, preferably 6.0-7.0.

Next, the dried, acid-treated waste oil fly ash sample is activated under CO₂ flow at 900-1050° C., preferably 925-1025° C., more preferably 950-1000° C., even more preferably 975-1000° C. The sample is heated for 30-90 min, preferably 45-75 min, and purged with CO₂ for 15-45 min, preferably 20-40 min. The pressure at which the CO₂ purging is performed is 1-5 bar, preferably 1.5-3 bar, while the CO₂ flow rate is kept at 0.5-2.0 L/min, preferably 1.0-1.5 L/min. The acid-treated waste oil fly ash sample is preferably not chemically modified by the CO₂ gas. The degree of burn-off after the acid chemical treatment and heated CO₂-infused physical activation, calculated based on measured weight difference between the untreated waste oil fly ash sample and the treated activated carbon, is 5-95%, preferably 10-75%, more preferably 20-60%.

In some embodiments the acid-treated activated carbon sample is further functionalized by attaching carboxylic groups and/or amine groups onto the surface of the activated carbon. To add carboxylic functional groups the acid-treated activated carbon sample is mixed with a second acid solution at a concentration of 50-200 g/L (g of activated carbon per volume of second acid solution), preferably 75-150 g/L, more preferably 100-125 g/L. The activated carbon-acid mixture is then heated at 80-120° C., preferably 85-100° C. for 3-6 h for a total reflux. In certain embodiments, the second acid solution contains a strong inorganic acid and is selected from nitric acid, sulfuric acid and hydrochloric acid. In one embodiment the second acid solution is nitric acid solution. In alternative embodiments, the second acid solution is an organic acid solution with examples as set forth herein.

To add amine functional groups, the same procedure is performed on the physicochemically treated activated carbon sample but with an ammonia or ammonium hydroxide solution. In one embodiment an activated carbon sample is functionalized with both carboxylic and amine groups. Accordingly, the reflux cycles with an acid solution and an ammonia solution are performed sequentially.

The amount of functional groups attached to the surface of the physicochemically treated activated can be determined, for example, by conventional elemental analysis techniques such as CHN analysis and mass spectrometric atomic spectroscopy and X-ray photoelectron spectroscopy (i.e. measuring nitrogen content to determine amine group content or oxygen content to determine carboxylic group content).

A physicochemically treated, non-functionalized activated carbon prepared according to a process described herein has a Brunauer-Emmett-Teller (BET) surface area of 30-400 m²/g, preferably 50-400 m²/g, 70-400 m²/g, more preferably 100-400 m²/g, 120-400 m²/g, 180-400 m²/g, even more preferably 250-400 m²/g, 300-400 m²/g, 350-400 m²/g, most preferably 375-400 m²/g. Comparatively, raw untreated waste oil fly ash has a BET surface area of 1-5 m²/g. Although the chemical treatment with acid alone can increase the surface area of waste oil fly ash by up to 20-25 times, it is the combination of the acid treatment and the heated CO₂ physical activation that is found to synergistically increase the surface area by up to 140-155 times.

On the other hand, a physicochemically treated activated carbon of the present disclosure that is further functionalized with amine groups, carboxylic groups or both has a BET surface area of 25-100 m²/g, preferably 30-90 m²/g, more preferably 35-85 m²/g, even more preferably 40-100 m²/g, 50-100 m²/g, 60-100 m²/g, 70-100 m²/g, 80-100 m²/g, 40-90 m²/g, 50-90 m²/g, 60-90 m²/g, 70-90 m²/g, 80-90 m²/g.

The physicochemical treatment of the waste oil fly ash sample is effective in removing non-carbonaceous impurities from the sample and thereby increasing the carbon content of the activated carbon. Accordingly, the activated carbon has a carbon content of at least 90%, preferably 90-95%, more preferably 95-99.9%. The oxygen content of the activated carbon after the physicochemical treatment is no more than 20%, preferably no more than 10%, more preferably no more than 5%, even more preferably no more than 2.5%. The sulfur content of the physicochemically treated activated is no more than 5%, preferably no more than 2.5%, more preferably no more than 1%. Other minerals, combined, are present only in trace quantities, constituting no more than 0.1% of the treated activated carbon, preferably no more than 0.05%, more preferably no more than 0.01%. The oxygen/carbon content of the physicochemically treated activated carbon is 0.005-0.2, preferably 0.01-0.1, more preferably 0.02-0.05. In certain embodiments, functionalization with amine groups results in a further slight increase in the carbon content of the activated carbon.

The activated carbon prepared in accordance with at least one of the processes described herein (with or without functionalization) has a total pore volume of 0.25-0.50 cm³/g, preferably 0.25-0.45 cm³/g, more preferably 0.30-0.40 cm³/g, with an average pore size of 40-100 Å, preferably 40-80 Å, more preferably 45-55 Å, even more preferably 50-55 Å (or 4-10 nm, 4-8 nm, 4.5-5.5 nm and 5.0-5.5 nm, since 1 Å=0.1 nm). In comparison, raw, untreated waste oil fly ash has a total pore volume of 0.02-0.05 cm³/g and an average pore size of 250-300 Å (25-30 nm).

At least 50% of the pores are in the diameter range of 40-100 Å, preferably 50-95%, more preferably 60-90%, most preferably 70-85%. The macropore/mesopore amount ratio of the physicochemically treated activated carbon is 0.001-0.05, preferably 0.002-0.02, more preferably 0.005-0.01. The micropore/mesopore amount ratio of the treated activated carbon is 0.002-0.1, preferably 0.005-0.05, more preferably 0.01-0.03.

Another embodiment of the present disclosure relates to a method for removing hydrogen sulfide from a gaseous stream or sample with the activated carbon provided herein. In one embodiment, the gas stream or gaseous sample is natural gas. As used herein, natural gas is composed primarily of methane (e.g. 80% or higher) and varying small amounts of C₂-C₅ alkanes, carbon dioxide, nitrogen and hydrogen sulfide. In accordance with the present disclosure, a natural gas stream is contacted with the modified, mesoporous activated carbon where the hydrogen sulfide in the gas stream is effectively adsorbed by the activated carbon. In one embodiment, the activated carbon is in a fixed mode. For example, the activated carbon can be affixed to the packed bed of an adsorption column and the natural gas stream is passed through the packed bed of activated carbon at 0.1-1.0 L/min, preferably 0.1-0.5 L/min at atmospheric or near atmospheric pressure and at ambient temperature (20-26° C.). In another embodiment, the gaseous stream or sample is an air sample or an effluent stream.

The amount of activated carbon required for the hydrogen sulfide removal in the method is 0.01-0.05 g per ppm of hydrogen sulfide present in the gas stream, preferably 0.02-0.04 g. The hydrogen sulfide adsorption capacity of the modified activated carbon in the method is 0.10-0.35 mg of hydrogen sulfide per gram of the activated carbon, preferably 0.20-0.35 mg, more preferably 0.25-0.35 mg. A spent activated carbon can also be regenerated and reused with a regeneration efficiency of 40-90%, preferably 55-90%, more preferably 80-90%.

In at least one embodiment, physicochemically treated activated carbon that is further functionalized with amine groups and has an average pore size of 50-55 Å is found to display the highest hydrogen sulfide capacity. The nitrogen content of the activated carbon functionalized with amine groups, as determined by any of the aforementioned elemental analysis techniques, is 5-30%, preferably 10-25%, more preferably 15-20%.

EXAMPLES

The following examples have been included to further describe protocols for synthesizing and characterizing modified, mesoporous activated carbon from waste oil fly ash, as well as methods for removing hydrogen sulfide from natural gas with the activated carbon. It should be noted that these examples have been included for illustrative purposes, and are not intended to limit the scope of the appended claims.

Due to low cost and large amount of unburned carbon, waste oil fly ash (OFA) can become an attractive choice material for the removal of H₂S from natural gas. In the following examples, it is shown that physicochemical treatments not only remove the mineral matter from ash but also result in a product with very high surface area. BET analysis shows an increase in the surface area from few square meters per gram to 375 m²/g. SEM images of the prepared activated carbon material show an increase in the number of micropores, well-developed particle size and porous structure due to activation with CO₂ at a high temperature. These examples demonstrate that proper combination of surface porosity and functional groups can lead to a suitable adsorbent for H₂S removal. Amine treatment after CO₂ activation leads to the formation of nitrogen functionalities on the carbon surface at the expense of reducing the surface area. The results are confirmed by BET analyses. On the other hand, HNO₃ functionalization of high surface area activated carbon has an adverse effect. The equilibrium capacity is reduced from 0.2966 mg/g to 0.1035 mg/g. The results indicate that the presence of more acidic functionalities on the surface reduces the H₂S adsorption efficiency from the gas mixture. Regeneration efficiency of the samples show that acid treatment followed by CO₂ activation at high temperature give the best option for removal of H₂S for OFA. In conclusion, the following examples show that physicochemical activation of a waste OFA sample can not only reduce a waste disposal problem and environmental pollution, but can also convert a waste product to a useful adsorbent.

Example 1 Materials

Raw waste oil fly ash (OFA) was collected from Rabigh power plant located in Saudi Arabia. It was dried overnight at 110° C. in the oven to remove moisture; sieved to 45 μm mesh; and stored in closed containers for further use. Elemental composition and porosity characterization results for untreated OFA are given in Table 1. The results show the presence of a high percentage of carbon in OFA (˜80%). Analytical grade nitric, phosphoric and sulfuric acids were supplied by Panreac Company, Spain. Ammonium hydroxide solution (25% w/w) with a density of 0.9 g/cm³ was obtained from Scharlau Company, Spain. Synthetic natural gas containing 50 ppm H₂S in methane was supplied by Saudi Gas Company. The Pyrex glassware was washed with demineralized water and dried in the oven at 105° C.

TABLE 1 Elemental composition and porosimetric characteristics of waste oil fly ash before treatment. Element Weight percentage (wt. %) C 77.40 S 7.10 O 9.32 Al 0.25 Mg 1.41 Ca 0.23 V 1.29 Ni 0.68 Cu 1.70 Zn 0.40 Si 0.08 Fe 0.14 BET Surface area (m²/g) 2.63 Micro pore surface Area (m²/g) 0 Pore Volume (cm³/g) 0.03 t-plot Micro pore Volume (cm³/g) 0

Example 2 Activation of Waste Oil Fly Ash

The activation process involves two steps: chemical treatment followed by physical activation with CO₂ at an elevated temperature. In a typical run, a sample of 10 g OFA powder was treated with 200 ml of an acid mixture in a round-bottom flask at 125° C. for 4 h under total reflux condition. Acid mixtures of different concentrations of HNO₃, H₃PO₄ and H₂SO₄ were used as shown in Table 2. The treated samples were filtered to obtain the solid residue. The residue was then rinsed repeatedly with distilled water to wash out the acid contents until a pH of spent acid has reached 6. The reason is that at higher temperatures desorption of oxygen functional groups and acid contents could create nascent sites on the surface of fly ash which will be more easily attacked by CO₂. In addition, removal of all acid will require high amounts of water. Finally, the solid residue was dried in an oven at 110° C. for 5 h. After that, the dried ash sample was subjected to physical activation with CO₂ at 990° C. in a programmable Lindberg Blue M tube furnace. The furnace was programmed to increase the temperature at a rate of 50° C./min. Once the required temperature was reached, a long horizontal tube (ID=1 cm) containing 6 g sample was heated to 990° C. for 1 hr and purged with 1 L/min of CO₂ for 30 min. The inlet pressure of CO₂ was set to 2 bar. Finally, the sample was cooled to room temperature and the product was kept in a desiccator for further characterization. The degree of burn off a (wt. %), is calculated using Eq. 1: α=(W _(i) −W _(f))/W _(i)  (Eq. 1) where W_(i)=initial mass of sample and W_(f)=final mass of sample, after CO₂ activation.

Example 3 Ammonia and Nitric Acid Treatment

A sample of 5 g of acid treated oil fly ash was mixed with 50 ml of NH₄OH solution in a round-bottom flask and heated at 90° C. for 4 h at total reflux. The mixture was then cooled, filtered and washed 5 times with 500 ml water. Filter cake was dried in an oven at 110° C. A similar procedure was repeated for the physicochemically treated OFA, where the sample was refluxed with HNO₃.

Example 4 Surface Area and Pore Size Determination

Micrometrics ASAP 2020 instrument was used to determine the BET surface area pore volume and other surface properties of sample like pore size distribution. A sample of 0.43 g treated OFA was degassed at 573K for 2 h under vacuum, and then N₂ gas was adsorbed-desorbed at 77K. Micro-pore volume was determined by the t-plot method [B. C. Lippens, J. H. De Boer, Studies on Pore Systems in Catalysts, J. Catal. 4(1965) 319-323—incorporated herein by reference in its entirety].

The activation of OFA with acid affects the porosity of ash compared to the raw material. The BET surface areas were determined from N₂ adsorption isotherms at 77 K. Table 2 summarizes the results of BET analyses after acid and physical treatment with CO₂. The acid treatment of OFA has leached out almost all major inorganic matter and increases the porosity. The surface area of acid treated OFA depends on the oxidation ability of the acid mixture. Based on O/H ratio in the chemical formula of the acid, the oxidation tendency of the acid increases according to the order HNO₃>H₂SO₄>H₃PO₄. Chemical treatment with mixtures of HNO₃/H₃PO₄ (without H₂SO₄) generates the highest surface area corresponding to 57.34 m²/g compared to single individual acid treatment. The incorporation of H₂SO₄ increases the degree of oxidation of the surface and makes the pore walls thinner thus easily damaged by HNO₃. As a result micropores get enlarged to mesopores and consequent diminishing of surface area values. For a combination of H₃PO₄/H₂SO₄, the acid treatment of ash experienced a very small weight loss of 0.1% as compared to other combinations and produces 0.66 m²/g BET surface area except for only H₂SO₄ for which surface area is 8.41 m²/g. This may be attributed to damage of pore structure or the formation of phosphate layer which covers the pore structure [B. S. Girgis, A.-N. a. El-Hendawy, Porosity development in activated carbons obtained from date pits under chemical activation with phosphoric acid, Microporous Mesoporous Mater. 52 (2002) 105-117—incorporated herein by reference in its entirety]. When ash was treated with a strong oxidizing mixture, consisting of only HNO₃/H₂SO₄ the weight loss was 91.3%. This shows strong oxidation conditions can destroy the carbon structure and pose a negative impact and may result in decreasing the surface area to a value less than that of raw OFA (i.e. 2.63 m²/g). Surface activation with the three acids at different volume ratios produced higher values of surface area compared to untreated OFA. Different acid compositions have different impact on weight loss of OFA. However, OFA treated with higher ratio of HNO₃/H₂SO₄ mixture showed a higher degree of weight loss in comparison with H₃PO₄/H₂SO₄. Raw OFA samples treated with a combination of the three acids generated the higher surface areas after CO₂ activation. Samples having surface area greater than 250 m²/g experienced a burn-off greater than 50%. This shows that the surface of OFA treated with more oxidizing mixture is more prone to the reaction of CO₂ with carbon that is essential in generating micro porosity, which leads to an increase of the surface area. Out of all different acid formulations covered in this study, OFA sample activated with 20% H₂SO₄, 40% HNO₃ and 40% H₃PO₄ (i.e. sample A4) generated the highest surface area (˜375.69 m²/g) after CO₂ activation as shown in Table 2. Consequently, OFA samples treated with this acid composition are further characterized for pore size distribution.

TABLE 2 Impact of physicochemical T = treatment on BET surface areas. Chemical Treatment CO₂ Activation BET Burn BET Weight surface off (%) surface Percentage of acids Loss area after CO₂ area No. H₂SO₄ HNO₃ H₃PO₄ (%) (m²/g) activation (m²/g) 1 0 0 100 7.3 1.40 12.2 49.60 2 0 40 60 41.5 57.34 37.8 120.59 3 0 100 0 56.7 19.02 32.6 104.61 4 20 40 40 56.3 4.05 55.1 375.69 5 40 0 60 0.1 0.66 0.8 33.07 6 40 20 40 12.5 4.14 35.1 195.52 7 40 60 0 91.3 1.23 56.4 281.22 8 100 0 0 20.5 8.41 3 72.15 9 0 0 0 — 2.63 5.61 7.49

FIGS. 2A and 2B show the pore size distribution for ash samples before and after treatment with acid mixture of 20% H₂SO₄, 40% HNO₃ and 40% H₃PO₄, respectively. The original ash sample has low pore size distribution and the pore volume increases with the increase in pore width. The total pore volume is related to the macropores for raw ash. Activation with acid mixture has increased the mesopore volume and decreases the macropores with an increase of total pore volume of 0.0257 cm³/g over the original ash sample. Further treatment with CO₂ at 990° C. has opened up more micro and mesopores while macropores has decreased with an average mesopore size of 50 Å. When the sample was treated with CO₂ at elevated temperatures micropores were generated with an average pore size of 50 Å compared to 109 Å obtained after acid treatment. Functionalization of selected samples with HNO₃ has increased the pore size to 54 Å but the surface area decreased to 39.03 m²/g from its original value of 375 m²/g.

The decrease in surface area could be attributed to the damage of the pore network by generating aggregation and alignment in the ash structure. Subsequent surface modification of selected sample with HNO₃ and NH₄OH enlarged some of the micropores to produce more mesopores as shown in Table 3. Surface area has been reduced from 375 m²/g of AC_(acid-CO2) to 80 m²/g and 39 m²/g during functionalization with NH₄OH and HNO₃, respectively. This is likely due to the decrease in pore volume. High surface area of AC_(acid-CO2) allows both (NH₄OH and HNO₃) functionalizing agents to penetrate inside and react readily with carbon particles. The total pore volume (Table 3) of AC_(acid-CO2-NH4OH) and AC_(acid-CO2-HNO3) is higher than that of AC_(acid-CO2) which is due to higher meso and macro porosity. Consequently, the average pore size decreased from 280 Å to 53 Å and 54 Å for AC_(acid-CO2-NH4OH) and AC_(acid-CO2-HNO3), respectively.

TABLE 3 Textural properties of waste oil fly ash sample A4 at different stages of activation and functionalization. t-plot micro- Total Average pore Pore Pore Volume, Volume, Size S_(BET) V_(micro), V_(0.99), *V_(meso) (4 v/A) Sample ID (m²/g) (cm³/g) (cm³/g) (cm³/g) (Å) Raw OFA 2.63 0    0.03  0.03  280  AC_(acid) 4.05 0.0010 0.0557 0.0547 109  AC_(acid-CO2) 375.69 0.0791 0.3002 0.2211 50 AC_(acid-CO2-NH4OH) 80.27 0.0672 0.3895 0.3223 53 AC_(acid-CO2-HNO3) 39.03 0.0600 0.3530 0.2930 54 *V_(meso) = V_(0.99) − V_(micro); AC = Activated Carbon

Example 5 Surface Morphology

A JEOL Scanning Electron Microscopy (Model JSM6400) in conjunction with Energy Dispersive Spectrophotometer (EDX) operated with 15 kV accelerating voltage was used to analyze the morphology of ash samples. Each sample was coated with a thin layer of gold before the analysis. Elemental composition of the sample was determined by analyzing the X-ray spectrum generated through spot analysis.

The EDX results of selected OFA treated samples are summarized in Table 4. The results show increase in oxygen to carbon ratio due to the leaching of some elements from raw OFA. Sulfur is reduced from 7.1 wt. % to 0.51 wt. % after acid treatment. The possible reasons are either oxidation of sulfur to SO₂ or the removal of free elemental sulfur during the washing step or a combination of these two processes. Sulfur dioxide is produced due to the strong interaction of HNO₃ with sulfate and pyrite sulfur in untreated OFA [R. Shawabkeh, M. J. Khan, A. a. Al-Juhani, H. I. Al-Abdul Wahhab, I. a. Hussein, Enhancement of surface properties of oil fly ash by chemical treatment, Appl. Surf. Sci. 258 (2011) 1643-1650—incorporated herein by reference in its entirety]. Subsequent CO₂ activation of acid treated ash breaks up some carbon bonding from the structure of ash and produces gaseous CO and carbon on the surface with oxygen chemisorbed on to it known as “surface-oxygen complex”. Under the reaction conditions, surface-oxygen complex may either become stable and inhibit further reaction of CO₂ with carbon surface by blocking the reaction sites or it decomposes to CO leaving a free surface carbon atoms available for further reaction [H. Marsh, F. R. Reinoso, Activated Carbon, 1st ed., Elsevier Ltd, New York, 2006, p. 243-245—incorporated herein by reference in its entirety].

TABLE 4 Average atomic composition of waste oil fly ash sample A4 after activation. Element AC_(acid) AC_(acid-CO2) AC_(acid-CO2-NH4OH) AC_(acid-CO2-HNO3) Carbon 79.13 95.71 96.79 83.61 (%) Oxygen 20.06 3.35 1.67 15.77 (%) Sulfur 0.51 0.93 1.54 0.62 (%) O/C 0.252 0.035 0.017 0.188 Weight 56.2 55.1 11 35 Loss (%) *AC = activated carbon and subscript shows the activation procedure.

The SEM images of FIGS. 3A-3E show that ash particles are composed of spheres, spheroids, and some agglomerates, and mostly are porous in nature. The size of ash particles varies from few to several microns. Similar values were also reported in previous studies [A. Mofarrah, T. Husain, C. Bottaro, Characterization of activated carbon obtained from Saudi Arabian fly ash, Int. J. Environ. Sci. Technol. (2013); C. M. López-Badillo, J. López-Cuevas, C. a. Gutiérrez-Chavarría, J. L. Rodríguez-Galicia, M. I. Pech-Canul, Synthesis and characterization of BaAl₂Si₂Os using mechanically activated precursor mixtures containing coal fly ash, J. Eur. Ceram. Soc. 33 (2013) 3287-3300—incorporated herein by reference in its entirety]. The SEM image of a raw waste oil fly ash sample (FIG. 3A) depicts the large cenospheres which are mixed with aggregates of particles. The pores are clearer and randomly located at the surface. Some pores of macro range can also be seen. After chemical treatment with the acid mixtures most of the metallic constituents are removed from the external surface and are at lower concentration inside the cenospheres, as shown in FIGS. 3B and 3C. These observations are consistent with the tabulated data obtained from EDX spot analysis (Table 4). An increase in oxygen content is due to oxidation of OFA while the percentage of sulfur is very low and all other minerals are removed. The presence of sulfur in the acid treated sample shows bonding with carbon inside the cenospheres. Some broken fly ash particles can also be seen due to the strong attack of concentrated mixture of acids. Physical activation has a significant influence on the porosity development and all the particles contain micro- and mesopores (FIG. 3C). Similar results were obtained for activated carbon prepared from coal fly ash [C. M. López-Badillo, J. López-Cuevas, C. a. Gutiérrez-Chavarría, J. L. Rodríguez-Galicia, M. I. Pech-Canul, Synthesis and characterization of BaAl₂Si₂O₈ using mechanically activated precursor mixtures containing coal fly ash, J. Eur. Ceram. Soc. 33 (2013) 3287-3300; Z. Lu, M. M. Maroto-Valer, H. H. Schobert, Catalytic effects of inorganic compounds on the development of surface areas of fly ash carbon during steam activation, Fuel. 89 (2010) 3436-3441—each incorporated herein by reference in its entirety]. Functionalizing the selected sample A4 with NH₄OH blocks some of the micropores and produces more mesopores, as shown in the FIG. 3D while the action of HNO₃ broke the activated carbon particles and severely affect the porosity, as can be seen in FIG. 3E and also from Table 3.

Spot analysis of selected sample was done to determine the percentage of carbon, oxygen and sulfur. Oxygen to carbon ratio shows the degree of oxidation after each activation process (Table 4). It is evident that the percentage of sulfur depends on the degree of oxidation as oxygen replaces sulfur on the surface of OFA [M. J. Khan, A. a. Al-Juhani, R. Shawabkeh, A. Ul-Hamid, I. a. Hussein, Chemical modification of waste oil fly ash for improved mechanical and thermal properties of low density polyethylene composites, J. Polym. Res. 18 (2011) 2275-2284—incorporated herein by reference in its entirety]. On the other hand, physical activation with CO₂ at a high temperature decreases oxygen to carbon ratio since all functional groups will leave the surface at high activation temperatures. Subsequent surface modification of selected sample #4 with either NH₄OH or HNO₃ shows different behavior against each functionalizing agent as shown in the Table 4.

The heating of AC_(acid-CO2) with ammonium hydroxide increases the basicity of activated carbon by introducing nitrogen containing groups to carbon surface. In addition, post treatment of AC_(acid-CO2) with HNO₃ leads to oxidation of carbon at the surface. The decrease in O/C ratio in AC_(acid-CO2-NH4OH) in comparison with AC_(acid-CO2) supports this fact. Post treatment of AC_(acid-CO2) with HNO₃ leads to oxidation of carbon at the surface. This is confirmed by the increase of O/C ratio from 0.035 to 0.188 as shown in Table 4. FTIR studies were performed to confirm the nitrogen and acidic containing functional groups on the surface of AC_(acid-CO2-NH4OH) and AC_(acid-CO2-HNO3), respectively. Due to the intense oxidation conditions during HNO₃ functionalization, the material has experienced a higher weight loss as compared to treatment of ACacid_(-CO2) with ammonium hydroxide.

Example 6 Hydrogen Sulfide Breakthrough Experiments

An adsorption column (length=12 cm, I.D=1 cm) connected with N₂ and CH₄/H₂S gas cylinders, as shown in the schematic diagram of FIG. 1, was used for breakthrough adsorption/desorption experiments. The column was packed with 2.0 g treated OFA sample. Initially, the column was purged with N₂ at a flow rate of 0.4 L/min for 15 min. Then a gas mixture containing 50 ppm H₂S and CH₄ as balance was introduced to the column at 0.4 L/min, 20% RH at 22° C. and 1 atm pressure. Exit H₂S concentration was continuously monitored by Multi RAE IR sensor every 2 s until reaching the saturation. Some gas samples were analyzed using GC-MS spectrophotometer to check the reliability of the gas meter.

After the completion of the adsorption cycle the valve was switched to N₂ at 1 atm to start the desorption cycle. The concentration of H₂S in the exit gas was measured continuously and stopped when the exit concentration of H₂S reaches zero ppm. The adsorption capacity was calculated from adsorption run using Eq. 2.

$\begin{matrix} {q = {\frac{C_{o} \times F}{M}{\int_{0}^{t_{e}}{\left( {1 - \frac{C}{C_{o}}} \right){dt}}}}} & \left( {{Eq}.\mspace{14mu} 2} \right) \end{matrix}$ were q=equilibrium adsorption capacity, mg/g; t_(e)=exhaust/saturation time of adsorbent; C_(o), C=Initial and concentration at time t, respectively, mg/cm³; F=gas mixture flow rate, L/min; M=mass of adsorbent, g; t=time, min.

The adsorption capacity after the desorption process was calculated using the area between adsorption and desorption curves. This capacity is the working capacity of the column. In this way we can define the regeneration efficiency (RE) of the column for the given dimension as follows:

$\begin{matrix} {{R\;{E(\%)}} = {{\frac{{Capacity}\mspace{14mu}{after}\mspace{14mu}{desorption}}{{Capacity}\mspace{14mu}{before}\mspace{14mu}{desorption}} \times 100\%} = {\frac{q_{de}}{q_{ad}} \times 100\%}}} & \left( {{Eq}.\mspace{14mu} 3} \right) \end{matrix}$

Small pore size, large surface area and basic surface functional groups play an important role in the adsorption of acidic gas molecules [R. T. Yang, Adsorbents: Fundamentals and applications, John Wiley & Sons, Inc., New Jersey, 2003—incorporated herein by reference in its entirety]. FIGS. 4-6 show the breakthrough curves of columns packed with different AC adsorbents. The adsorbents were produced through different physicochemical activations and surface modifications of OFA. Breakthrough times and equilibrium capacities of raw and activated OFA samples were calculated and the results are given in Table 5. OFA activated with the mixture of three acids and raw OFA show approximately the same (very short) breakthrough times (Table 5), but the adsorption and desorption curves are quite different from each other as shown in FIG. 4. It is expected that H₂S is strongly attached to raw OFA surface due to the strong host/guest attractions in the presence of mineral matter.

However, after acid treatment of raw OFA sample, the surface is oxidized through formation of acidic carbonyl functional groups. The oxidized surface would likely give a rise to repulsive surface forces relative to H₂S. Hence, the tendency of the adsorption-desorption process would be almost physical for AC_(acid) compared to raw OFA sample (FIG. 4). The H₂S breakthrough test with AC_(acid-CO2) shows that the adsorbed amount of H₂S has increased (FIG. 6). The same figure also shows desorption curve. The desorbed amount can be viewed as physically attached to the surface of OFA during the adsorption step, while retained amount can be considered as chemically adsorbed. According to BET analysis shown in the previous section, the surface area, micro pore volume and total basicity increases and all acid groups vanished at higher temperature during CO₂ activation. The decrease in the desorbed amount can be explained on the basis of the Dubinin theory of micropore filling theory. The theory which states that pores of smaller diameter will adsorb more solute at lower concentration because of higher adsorption potential exerted by the walls [W. Feng, S. Kwon, E. Borguet, R. Vidic, Adsorption of hydrogen sulfide onto activated carbon fibers: Effect of pore structure and surface chemistry., Environ. Sci. Technol. 39 (2005) 9744-9—incorporated herein by reference in its entirety].

FIG. 7 shows the H₂S breakthrough results for NH₄OH and HNO₃ treated AC_(acid-CO2) samples. Ammonium hydroxide treatment of previously developed activated carbon, i.e. AC_(acid-CO2) gives almost the same result as AC_(acid-CO2). although the surface area is reduced for AC_(acid-CO2-NH4OH) sample. This can be attributed to the nitrogen functionalities attached to the surface of carbon. As compared to NH₄OH treatment, HNO₃ treatment again increases the oxidation of the surface, which in turn reduces the equilibrium capacity of H₂S. More acidic surface of adsorbent inhibits the dissociation of H₂S, hence decreases the concentration of hydrogen sulfide ions. Low concentration of these ions promotes the formation of high-valent sulfur compounds like SO₂ and sulfuric acid which leads to low H₂S removal capacity. On the other hand, adsorbent surface with pH in the basic range favors the dissociation of H₂S to hydrogen sulfide ions, i.e., HS⁻. These ions are oxidized to form sulfur polymers having ring or chain like shape yielding higher H₂S removal capacities [W. Feng, S. Kwon, E. Borguet, R. Vidic, Adsorption of hydrogen sulfide onto activated carbon fibers: Effect of pore structure and surface chemistry., Environ. Sci. Technol. 39 (2005) 9744-9; M. Abe, K. Kawashima, K. Kozawa, H. Sakai, K. Kaneko, Amination of activated carbon and adsorption characteristics of Its aminated surface, Langmuir. 16 (2000) 5059-5063; F. Adib, A. Bagreev, T. J. Bandosz, Adsorption/Oxidation of hydrogen sulfide on nitrogen-containing activated carbons, Langmuir. 16 (2000) 1980-1986; T. J. Bandosz, On the adsorption/oxidation of hydrogen sulfide on activated carbons at ambient temperatures., J. Colloid Interface Sci. 246 (2002) 1-20—each incorporated herein by reference in its entirety].

Regeneration efficiency was calculated for different AC samples using Eq. 3 and the results are given in Table 5. It is essential to effectively regenerate loaded adsorbent in industrial applications (such as pressure swing adsorption) for economic feasibility of the process. Regeneration efficiency is an indicator of reusability of any sorbent. Table 5 shows that regeneration efficiency of raw OFA sample is ˜83%. Both the equilibrium capacity and regeneration efficiency of AC_(acid) are the lowest compared to all other samples. The reason behind this observation is already mentioned that this sample possess repulsive functional groups on its surface. However, activation of this sample by CO₂ at high temperature increases both the capacity and regeneration efficiency, which can lead to a potential commercial application.

TABLE 5 Equilibrium capacity and breakthrough time of hydrogen sulfide after adsorption tests. Adsorption Break Equilibrium caspacity left Regeneration Through Sample capacity after desorption efficiency Time type (mg/g) (mg/g) (%) (sec) Raw OFA 0.0231 0.0192 83.41  6 AC_(acid) 0.0088 0.0036 40.96  8 AC_(acid-CO2) 0.2966 0.2639 89.00 770 AC_(acid-CO2-NH4OH) 0.3001 0.2594 86.43 450 AC_(acid-CO2-HNO3) 0.1035 0.0573 55.34  36

Example 7 Comparisons with the Prior Art

The detailed differences between the present disclosure and the pertinent prior art are given in Table 6. Specifically, the differences in the final surface area obtained for each modification and the adsorption capacity are summarized in Table 6. All references cited in Table 6 are incorporated herein by reference in their entireties.

TABLE 6 Comparisons of various activated carbon from the prior art with the present disclosure. Adsorbent Modification Conclusions Reference Commercial i. Impregnation with strong i. Adsorption depends Y. Elsayed, M. coconut-based base (NaOH) and an on the basic functional Seredych, A. activated oxidant (HCl); groups on the surface Dallas, T. J. carbon ii. 2 concentrations of H₂S of carbon. Bandosz, (10 ppm and 1000 ppm) ii. Moisture content in Desulfurization were examined. H₂S or on the surface of air at high of the adsorbent and low H₂S enhances adsorption concentrations, capacity. Chem. Eng. J. 155 (2009) 594-602. Coconut-based i. Impregnation with 3 i. Carbon impregnated H. S. Choo, L. C. activated types of alkaline solutions with K₂CO₃ exhibited Lau, A. R. carbon (NaOH, KOH, K₂CO₃) at highest adsorption Mohamed, K. T. different ratios; capacity. Lee, Hydrogen ii. Sorption tests were ii. As the concentration sulfide performed using simulated of H₂S increases, the adsorption by biogas prepared by mixing adsorption capacity of alkaline CH₄, CO₂ and H₂S, i.e. the activated carbon impregnated 50% CO₂ with varying decreases. coconut shell concentrations of H₂S activated (1000-5000 ppm and the carbon, J. Eng. balanace is CH₄. Sci. Technol. 8 (2013) 741-753. Bituminous i. Coal was first oxidized Modifications of A. Bagreev, J. coal-based with air and then pyrolized bituminous coal-based Angel activated under N₂ and CO₂ flow. activated carbon with Menendez, I. carbon Pyrolized sample was N₂-containing species Dukhno, Y. further oxidized with 50% increases the H₂S Tarasenko, T. J. HNO₃ for 4 h and then adsorption capacity Bandosz, washed with water. than unmodified Bituminous Oxidized samples were carbon. coal-based divided into two portions, activated i.e. one portion was treated carbons with melamine suspension modified with while the other was treated nitrogen as with a melamine-urea adsorbents of suspension followed by hydrogen heat treatment at 850° C. sulfide, Carbon. ii. Moist air stream 42 (2004) 469-476. containing 0.3% (3000 ppm) H₂S was used in adsorption experiments. Activated i. Different schemes were ACFs with higher W. Feng, S. carbon fibers used to modify the surface surface area gave high Kwon, E. (ACFs) of carbon and 3 adsorbents adsorption capacity and Borguet, R. manufactured were prepared by drying retention of sulfur Vidic, by samples under N₂ flow at which further enhances Adsorption of polymerization 140° C. for 2 h followed by with heat treatment of hydrogen sulfide of phenol and heating under N₂ at 900° C. carbon. onto activated formaldehyde for 4-6 h and then cooled carbon fibers: to room temperature. Also, Effect of pore samples were dried under structure and N₂ and then oxidized with surface O₂ at 200° C. for 2 h. chemistry., ii. N₂ gas stream carrying Environ. Sci. 200 ppm H₂S was used in Technol. 39 adsorption tests (2005) 9744-9. Activated i. Carbon samples were Acidic environment F. Adib, A. carbon made washed to remove water- promotes the formation Bagreev, T. J. from different soluble impurities and of sulfur oxide and Bandosz, raw materials dried at 120° C. sulfuric acid while the Adsorption/ (Bituminous ii. Samples were oxidized basic environment Oxidation of coal, coconut with 15 M HNO₃ with a promotes the formation hydrogen sulfide shell, wood ratio of 5 ml acid to 1 g of of elemental sulfur. on nitrogen- and peat) carbon. containing iii. After oxidation, one activated portion of the sample was carbons, set aside while the other Langmuir. 16 portion was further treated (2000) 1980-1986. with ammonium persulfate solution. iv. N₂ gas stream carrying 3000 ppm H₂S was used in adsorption tests. Waste oil fly Treatment with a mixture High surface area and The present ash (OFA) was of sulfuric, nitric and high adsorption disclosure. used. OFA phosphoric acids followed capacity were obtained. with carbon by thermal treatment with content of CO₂ at high temperatures more than 80% up to 990° C., then treatment and the rest is with ammonium mainly metal hydroxide. oxides and sulfur.

Thus, the foregoing discussion discloses and describes merely exemplary embodiments of the present invention. As will be understood by those skilled in the art, the present invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. Accordingly, the disclosure of the present invention is intended to be illustrative, but not limiting of the scope of the invention, as well as other claims. The disclosure, including any readily discernible variants of the teachings herein, defines, in part, the scope of the foregoing claim terminology such that no inventive subject matter is dedicated to the public. 

The invention claimed is:
 1. A process for preparing an acid-treated activated carbon material, comprising: refluxing a waste oil fly ash powder with a first acid solution comprising at least one acid selected from the group consisting of sulfuric acid, nitric acid and phosphoric acid to form a solid residue; rinsing the solid residue with water until the water has a pH of at least 6; and heating the solid residue at 900-1050° C.; and purging the solid residue in a flow of carbon dioxide to from the activated carbon material.
 2. The process of claim 1, wherein the first acid solution comprises two acids selected from the group consisting of sulfuric acid, nitric acid and phosphoric acid at a volume ratio of 1-13:1-13.
 3. The process of claim 1, wherein the first acid solution comprises sulfuric acid, nitric acid and phosphoric acid at a volume ratio of 1-8.5:1-8.5:1-8.5.
 4. The process of claim 1, wherein the refluxing is carried at 100-150° C. for 3-6 h and at a concentration of 10-100 g of the waste oil fly ash powder per liter of the first acid solution.
 5. The process of claim 1, wherein the heating is carried out for 30-90 min.
 6. The process of claim 1, wherein the purging is carried out for 15-45 min, at 1-5 bar and at a carbon dioxide flow rate of 0.5-2.0 L/min.
 7. The process of claim 1, further comprising: contacting and refluxing the activated carbon material with an ammonia solution.
 8. The process of claim 7, wherein the contacting and the refluxing are carried out at 80-120° C. for 3-6 h and at a concentration of 50-200 g of the activated carbon material per liter of the ammonia solution.
 9. The process of claim 1, wherein the refluxing, the heating and the purging increase the BET surface area of the activated carbon material, increase the total pore volume of the activated carbon material and reduce the average pore size of the activated carbon material. 